1. Field of the Invention
This invention relates to electron beam systems and, more particularly, to neutralizing the charge in such systems.
2. Description of Related Art
Manufacturing semiconductor devices often requires the use of charged particle beams or electron beam (e-beam) systems. The manufacturing process is typically performed using lithographic processes followed by various subtractive (etch) and additive (deposition) processes. Examples of additive processes include deposition of pre-metal dielectric, inter-level dielectric, passivation, and metal layers. Subtractive processes typically involve focusing an electron beam onto the surface of a semiconductor substrate to generate patterns on the substrate. Substrates can include semiconductor wafers, mask plates, printed circuit boards (PCBs), liquid crystal display (LCD) panels, and other various devices. Pattern generation, in which, e.g., CAD patterns are transferred to the surface of the device, can be accomplished using a variety of processes including photolithography, ion beam lithography, and electron beam lithography. In the case of a charged particle system (e.g., e-beam), a precise beam of charged particles is directed to a specific point on the surface of a material, such as photoresist, that is sensitive to relatively small amounts of energy to selectively initiate a response by the material at that point. The beam is selectively directed to various points on the device to create a desired pattern.
An example of a charged particle system is an e-beam system. The e-beam lithography process typically includes first programming a desired image pattern into an e-beam exposure tool. A substrate, such as a silicon wafer, that has been coated with an e-beam sensitive material, such as photoresist, is then mounted within a writing chamber. Once the substrate is secured, the electron beam is selectively brought in contact with the resist, based on the programmed pattern, to create the desired pattern on the resist. The resist can then be processed, e.g., developed and selectively removed, to expose portions of the underlying layer, which can then be etched to form the desired pattern in the substrate.
As seen, in order to accurately transfer the programmed image pattern to the substrate, the pattern must be accurately and precisely reproduced on the photoresist. Various factors can affect the accuracy of the pattern transferred to the resist, including the resolution of the e-beam, the precision of the e-beam tool, the size and density of the features on the pattern, and the placement accuracy of the pattern. The placement accuracy can be adversely affected by charge build-up on the resist. Although, resists are generally non-conductive, the effects of electrons impinging on the resist can cause this charge build-up.
As the electrons in the e-beam or other charged particles impinge and penetrate the photoresist, 1) secondary electrons are emitted and scattered away from the substrate, and 2) the penetrating charged particles undergo an energy loss scattering process, which brings them to rest on the photoresist, where they can remain for several hours after exposure. These two effects result in an accumulation of electrostatic charge and generation of electric fields about these areas. Thus, as the e-beam is scanned or positioned over an adjacent portion of the photoresist to continue defining the desired pattern, the electric field can deflect the e-beam from the path of the programmed pattern and prevent the desired spot on the resist to be selected. The result is a feature that has errors in position and/or size because more or less of the resist is exposed than what was intended. This may occur with either positive or negative polarity resist. For example, with positive resist (in which exposed portions are made more soluble and removed after development), a deflected e-beam may expose more resist than desired, thereby resulting in a longer and/or wider feature. With a negative resist (in which exposed portions are polymerized and unexposed portions are removed), a deflected e-beam may again expose more resist than desired, this time resulting in a shorter and/or narrower feature.
In addition to manufacturing semiconductor devices, electron or charged particle beams are also used to test and analyze or observe such devices. Similar to the above-described effects, the electron beam can be deflected from an intended point or area of the device due to an electric field generated by surface electrons on the device. Also, for example, when analyzing or observing an insulator (non-conducting) material, electrostatic charging can cause errors when measuring secondary electrons emitted from the surface resulting from the impinging e-beam. This can cause measurement inaccuracies, which is particularly important in electron beam testing systems utilized to determine continuity, leakage and open circuits of Liquid Crystal Display (LCD) panels and (PCB) printed circuit boards. In this application, periodic discharging of the panel under test is required. Thus, the charging of the substrate can be the cause of various errors associated with the manufacture and testing of a semiconductor device or other types of devices. These adverse effects become more significant as the devices continue to decrease in size, with critical dimensions decreasing and density increasing.
One method to neutralize this charge accumulation is to deposit a conductive layer on the substrate to remove the surface charge. However, this additional step increases the time and costs for manufacturing the device, as well as possibly introducing defects and increasing the complexity of the device, and it is not easily applicable to the LCD panel or PCB testing.
Accordingly, it is desired to be able to utilize electron or charged particle beams on semiconductor devices and other types of devices without the adverse effects of trapped charge discussed above.
The present invention introduces a relatively low pressure ionized gas above the surface of a substrate or device to neutralize charge created on the surface by an impinging electron or charged particle beam. Argon, helium, or other suitable gases can be used.
In one embodiment, the gas is introduced throughout the vacuum chamber. A radiation source, such as a vacuum ultraviolet (VUV) radiation lamp or a soft x-ray generator, directs a light or x-ray beam across and above the surface of the substrate. The beam ionizes the gas above the substrate to absorb or neutralize charge accumulated on the surface from the impinging electron beam (e-beam). In some embodiments, the gas is localized or confined to an area over the substrate. In other embodiments, the light or x-ray beam is directed towards the surface of the substrate, either directly by the radiation source or deflected, such as with a perforated mirror. The beam impinging on the substrate creates the photoelectric effect and surface-induced conductivity, which augments the effect of the ionized gases, thereby reducing the neutralization time. For some applications, this embodiment allows the reduction of the gas pressure to zero (no gas) and the utilization of only the photoelectric effect and/or surface induced conductivity to discharge the substrate.
In a particular embodiment of the invention, with a 1 m distance between the electron source and the substrate and utilizing a 10 KeV electron beam, if argon is used, the gas pressure can be up to approximately 1 mTorr, and if helium is used, the pressure can be up to approximately 10 mTorr.
This invention will be more fully understood in light of the following detailed description taken together with the accompanying drawings.